Low dew point gas generator cooling system

Abstract

A drying system is disclosed for drying wet gases, typically produced by a gas generator to dew points less than 32 DEG F. The system uses a refrigeration cycle having a single compressor to dry the wet gas in a first evaporator while simultaneously defrosting the ice buildup on a second evaporator. The cycle then switches from the first evaporator to the second evaporator after the second evaporator is defrosted. The switching is preceded by a changeover phase in which cooled refrigerant is pulsed to the defrosted evaporator and both evaporators go on line for a short time before the system phase change occurs thus assuring control of the dried gas temperature while avoiding shock to the refrigeration system.

Having thus defined the invention, it is claimed: 
     
       1. A method for producing from a wet gas stream a continuous stream of dried gas having a dew point less than about 20° F. comprising the steps of: a) providing i) a single compressor for compressing a refrigerant gas to produce a compressed, heated refrigerant gas stream, ii) a condenser for liquefying said compressed heated refrigerant gas stream to produce a cooled refrigerant stream, iii) first and second evaporator coils with associated expansion valves, and iv) valves for porting said refrigerant streams between said evaporators and said wet gas stream over said evaporators;   b) repeatedly performing a periodic, refrigeration cycle in which i) during a first phase of said cycle, said wet gas is passed over said first evaporator which functions as an active evaporator through which said cooled refrigerant stream passes for drying said wet gas passing over said first evaporator while a portion of said heated compressed refrigerant stream is passed through said second evaporator which is in an inactive state for defrosting same and ii) during a second phase of said cycle, said wet gas is ported over said second evaporator acting as an active evaporator through which said cooled, refrigerant stream passes for drying said wet gas while a portion of said heated compressed refrigerant in passed through said first evaporator functioning as an inactive evaporator for defrosting same;   c) during each cycle phase metering a portion of said heated compressed refrigerant stream into said cooled refrigerant stream ported into said active evaporator, the metering rate of said heated refrigerant stream being periodically reduced from the beginning to the end of each cycle phase as ice accumulates on said active evaporator whereby said wet gas is continuously dried to a generally constant dew point throughout each portion of the cycle.   
     
     
       2. The method of claim 1 further including in step (c) metering said heated compressed refrigerant stream into said inactive evaporator at decreasing flow rates so that the internal temperature of the coils of said inactive evaporator does not increase to high temperatures prior to cycling said inactive evaporator to an active evaporator. 
     
     
       3. The method of claim 1 further including the step of performing a changeover phase before switching from one numbered phase to the other by i) stopping said flow of heated refrigerant stream through said inactive evaporator and ii) thereafter pulsing a portion of said cooled refrigerant stream through said inactive evaporator while maintaining the flow of said cooled refrigerant stream through said active evaporator for a first set time period at the end of which said inactive evaporator becomes active by porting said wet gas stream over and said cooled compressed refrigerant stream through said inactive evaporator while said active evaporator becomes inactive by receiving only a portion of said heated compressed refrigerant stream ported therethrough while said wet gas is no longer in contact therewith whereby said cycle is switched between numbered phases without causing variation in the dried temperature of the cooled gas. 
     
     
       4. The method of claim 3 wherein the time duration of said pulses in said changeover phase is periodically increased until said pulse becomes continuous at which time said wet gas no longer contacts the evaporator which was previously active, and at a second shorter time period within said changeover phase causing said wet gas to contact the inactive evaporator whereby both evaporators are on line prior to switching from one numbered phase to the other to maintain the drying temperature of the wet gas. 
     
     
       5. The method of claim 1 further including the step of directing a portion of said dried gas over the coils of the inactive evaporator during each numbered phase of said refrigeration cycle whereby ice is removed not only by heat transfer from within the coils of the inactive evaporator but also by moisture pick up from the dried gas circulating about the exterior of the coils of the inactive evaporator. 
     
     
       6. The method of claim 4 further including the step of directing a portion of said dried gas over the coils of the inactive evaporator during each numbered phase of said refrigeration cycle whereby ice is removed not only by heat transfer from within the coils of the inactive evaporator but also by moisture pick up from the dried gas circulating about the exterior of the coils of the inactive evaporator. 
     
     
       7. The method of claim 6 further including in step (c) metering said heated compressed refrigerant stream into said inactive evaporator at decreasing flow rates so that the internal temperature of the coils of said inactive evaporator does not increase to high temperatures prior to cycling said inactive evaporator to an active evaporator. 
     
     
       8. The method of claim 7 wherein the numbered phase said cycle is controlled by sensing the temperature of the refrigerant stream immediately downstream of each evaporator and controlling the metering of said heated compressed refrigerant stream in response thereto. 
     
     
       9. The method of claim 8 further including the step of controlling the pulsing of said cooled, compressed refrigerant stream and the porting of said wet gas stream from one evaporator to the other in response to the sensed temperature resulting in a stable and responsive control system. 
     
     
       10. The method of claim 5 further including the step of venting said dried gas after it has contacted the coils in said inactive evaporator. 
     
     
       11. The method of claim 9 wherein said evaporator has coils contained in a sealed enclosure open to a water sump and an exit port on one side and an inlet port on the other side, said wet gas passing over said coils in said enclosure to become dried gas and thereafter contacting said water sump before passing through said exit port without experiencing any adverse increase in dew point. 
     
     
       12. The method of claim 1 further including at least a third evaporator, said refrigeration cycle proceeding to activate only one of said evaporators in sequential order, the other evaporators being inactive and remaining in said defrost mode until said changeover phase is selected. 
     
     
       13. A method for producing a gas dried to a low dew point suitable for use as the furnace atmosphere in an industrial heat treat process, said method comprising the steps of: a) reacting air with a combustible hydrocarbon gas at a set ratio in a gas generator to produce a wet gas having a desired gas composition including water vapor and immediately drying said wet gas to a dew point of about 100° F.;   b) providing i) a compressor for compressing a refrigerant gas to produce a compressed, heated refrigerant gas stream, ii) a condenser for liquefying said compressed heated refrigerant gas stream to produce a cooled refrigerant stream iii) first and second evaporator coils, each having associated expansion valves, and iv) valves for porting said refrigerant streams between said evaporators and said wet gas over said evaporators;   c) repeatedly performing a periodic, refrigeration cycle in which i) during a first phase of said cycle wet gas is passed over said first evaporator functioning as an active evaporator through which said cooled refrigerant stream passes for drying said wet gas to dew points less than 32° F. while a portion of said heated compressed refrigerant stream is passed through said second evaporator functioning as an inactive evaporator for defrosting same, and ii) during a second phase of said cycle said wet gas is passed over said second evaporator coil acting as an active evaporator through which said cooled, refrigerant stream passes for drying said wet gas to dew points less than 32° F. while a portion of said heated, compressed refrigerant is passed thorough said first evaporator for defrosting same whereby the sensible heat of said wet gas is utilized by said refrigeration cycle to defrost said inactive evaporator.   
     
     
       14. The method of claim 13 further including the step, during each cycle phase, of metering a portion of said heated, compressed refrigerant stream into said steam of cooled refrigerant at a decreasing rate during the time said cooled refrigerant stream is utilized by said active evaporator whereby the efficiency of said active evaporator is increased to compensate for ice buildup on the evaporator coils to maintain the dew point of said dried gas at a set value while simultaneously metering said heated compressed refrigerant stream into said inactive evaporator at decreasing flow rates so that the internal temperature of the coils of said inactive evaporator does not increase to high temperatures prior to switching said inactive evaporator to an active evaporator. 
     
     
       15. The method of claim 14 further including the step of performing a changeover phase before switching from one numbered phase to the other by i) stopping said flow of heated refrigerant stream through said inactive evaporator and ii) thereafter pulsing a portion of said cooled refrigerant stream through said inactive evaporator while maintaining the flow of said cooled refrigerant stream through said active evaporator for a first set time period at the end of which said inactive evaporator becomes active by porting said wet gas stream over and said cooled compressed refrigerant stream through said inactive evaporator while said active evaporator becomes inactive by receiving only a portion of said heated compressed refrigerant stream ported therethrough while said wet gas is no longer in contact therewith whereby said cycle is switched between numbered phases without causing variation in the dried temperature of the cooled gas. 
     
     
       16. The method of claim 15 wherein the time duration of said pulses in said changeover phase is periodically increased until said pulse becomes continuous at which time said wet gas no longer contacts the evaporator which was previously active, and at a second shorter time period within said changeover phase causing said wet gas to contact the inactive evaporator whereby both evaporators are on line prior to switching from one numbered phase to the other to maintain the drying temperature of the wet gas. 
     
     
       17. The method of claim 16 further including the steps of collecting said dried gas in a storage tank prior to use in said heat treat process, directing a portion of said dried gas from said storage tank to pass over the coils of said inactive evaporator whereby removal of ice from the inactive evaporator during defrost is further assisted. 
     
     
       18. The method of claim 17, wherein said dried gas after passing over the ice buildup on the coils of said inactive evaporator is vented from the system. 
     
     
       19. Apparatus for drying wet gases having dew points in excess of about 40° F. to dry gases having a dew point less than 32° F. comprising: a) a refrigeration system including a i) compressor for compressing a refrigerant gas to a heated, compressed refrigerant gas, ii) a condenser in fluid communication with said compressor for condensing said heated refrigerant gas to a cooled, condensed refrigerant, iii) first and second evaporators with associated expansion valves for drying said wet gas, and iv) refrigerant valve means for selectively porting said heated compressed refrigerant and said condensed refrigerant to said evaporators;   b) a cooling tank having first and second compartments, each with an inlet and outlet and insulated from one another and in which said first and second evaporators, respectively, are disposed, each compartment having cooling tank valve means controlling which compartment and its associated evaporator receives said wet gas;   c) control means regulating said cooling tank valve means and said refrigeration valve means for repeatedly performing a periodic refrigeration cycle in which i) during a first phase of said cycle wet gas is passed over said first evaporator functioning as an active evaporator through which said cooed refrigerant stream passes for drying said wet gas to a dew point less than 32° F. while a portion of said heated compressed refrigerant stream is passed through said second evaporator functioning as an inactive evaporator for defrosting same, and ii) during a second phase of said cycle said wet gas is passed over said second evaporator coil acting as an active evaporator through which said cooled, refrigerant stream passes while a portion of said heated, compressed refrigerant is passed through said first evaporator for defrosting same whereby the sensible heat of said wet gas is utilized by said refrigeration cycle to defrost said inactive evaporator.   
     
     
       20. Apparatus of claim 19 further including said refrigeration system having a refrigerant conduit containing said expansion valves between said condenser and said evaporators; a defrost conduit downstream of said compressor and upstream of said condenser in fluid communication with said first and second evaporators downstream of each evaporator's expansion valve; said refrigerant valve means including refrigerant valves in said refrigerant conduit and defrost valves in said defrost conduit, all regulated by said control means; said control means effective to port said heated compressed refrigerant at decreasing rates through said inactive evaporator for defrosting same while metering a portion of said heated compressed refrigerant into said cooled refrigerant of said active evaporator at a rate which decreases over the time period said active evaporator is cooling said wet gas said decreasing rate being correlated to the ice buildup on the coils of said active evaporator so that the dew point of the dried gas remains relatively constant throughout the cycle. 
     
     
       21. Apparatus of claim 20 further including a hot regulating valve in said defrost conduit upstream of said defrost valves for controlling the temperature of said heated refrigerant gas in said defrost conduit to a set temperature. 
     
     
       22. Apparatus of claim 21 further including temperature means sensing the temperature of said refrigerant after said refrigerant leaves said active evaporator, said control means metering said heated compressed refrigerant to said active evaporator in response to the temperature sensed by said temperature means. 
     
     
       23. Apparatus of claim 22 wherein said control means causes a changeover phase before switching from one numbered cycled phase to the other by regulating said defrost valves to stop the flow of said heated refrigerant stream through said inactive evaporator and thereafter pulsing a portion of said cooled refrigerant stream through said inactive evaporator while maintaining the flow of said cooled refrigerant stream through said active evaporator for a set time period before causing said cycle to switch from one numbered phase to another whereby said cycle is switched between numbered phases without causing variation in the dried temperature of the cooled gas. 
     
     
       24. Apparatus of claim 23 wherein said control means causes the time duration of said pulses in said changeover phase is periodically increased until said pulse becomes continuous at which time said wet gas no longer contacts the evaporator which was previously active, and at a second shorter time period within said changeover phase causing said wet gas to contact the inactive evaporator whereby both evaporators are on line prior to switching from one numbered phase to the other to maintain the drying temperature of the wet gas. 
     
     
       25. Apparatus of claim 24 further including a buffer tank in fluid communication with said outlets of said cooling tank for storing a quantity of said dried gas, said buffer tank in valved fluid communication with said inlets of said cooling tank, said control means effective to cause a portion of said dried gas in said buffer tank to pass through that inlet in fluid communication with an inactive evaporator for removing ice buildup. 
     
     
       26. Apparatus of claim 25 further including said cooling tank having a common sump beneath said compartments for collecting water condensed from said evaporator coils; a partition extending between said compartments into said sump dividing said cooling tank into two separate passages beneath said evaporators and vent valve means in fluid communication with said cooling tank outlets for venting gas from said outlet; said vent valve means controlled by said control means to vent only dried gas from said buffer tank after said dried gas passes over said inactive evaporator.